unit usually regers to a percentage of bankroll.
lets say for example you have 1000$ to wager with, or that was the maximum amount of money that you were comfortable in losing. That would be called your bankroll.
Most people advise betting 1% to 5% on each play of your bankroll.
In this example this would mean your plays would be 10$ and 50$.
When you post on the forum, you dont really want to talk about money, so units are a more favorable term to use.
In this persons example
1 unit = 10$
2 units = 20$
3 units = 30$
4 units = 40$
5 units = 50$